The Sept. 8, 2000, News-Times story, County gets update on marijuana fight includes a description by Lincoln County Forest Enforcement Officer Chuck Bergman, a recent candidate for Sheriff. Bergman describes the practice of discovering marijuana patches in the woods during aerial surveillance. Bergman claims that he has had special training which allows him to spot the marijuana because of its distinctive color.
This claim is the basis for a long-used hoax by law enforcement which allows them to protect their informants, otherwise known as "snitches." The article mentions Bergman's use of "phoned-in tips that earn the tipster a reward." According to Bergman, "the informant's name is kept anonymous." Of course it is kept secret; no snitch is going to go into court and publicly testify in support of the cop's application for the search warrant. A warrant is necessary before the police can enter private property and seize the pot. If they go in without the warrant, they are in violation of the accused's constitutional right of privacy; the pot cannot be used as evidence - because it was seized illegally.
The way the cops avoid being caught violating defendants' Sixth Amendment constitutional rights to confront their accusers is to fly over the site of the patch that has been located by their snitch, hover, take pictures, pick out the plants under a magnifying glass, and claim in court to have made the discovery in the course of routine overflights.
The clear proof of the absurdity of police claims of discovering individual plants and small marijuana patches during aerial surveillance can be shown by anyone using binoculars while riding as a passenger in a car at 40 MPH. Attempt to identify particular plants at a distance of several hundred feet on a brushy hillside. No one, no matter what kind of training, can make such identifications reliably. As to the unique coloration of marijuana, any botanist will tell you that plant color varies depending on what nutrients the plants obtain.
Aerial surveillance for marijuana really began in Humbolt County, California, in the mid to late 1960's. There, on the brown hills, the green of irrigated marijuana patches stood out like flags, easily spotted from the air. The technique was credible under those conditions, and it was unchallenged when it was imported into the Northwest for the purpose of getting search warrants while protecting snitches.
The practice of using paid "informants" as unofficial police agents to perform illegal searches (violations of privacy rights which would invalidate the evidence if done by salaried officers) is firmly established and accepted by most courts in the US. There are occasional but futile challenges by defense lawyers; currently there are bills before Congress sponsored by right-wing politicians which would explicitly permit the admission of such evidence.